This invention generally relates to single antenna transponders. More particularly, the invention relates to extremely compact single antenna dual frequency transponders which may be incorporated into a wrist instrument or a card for security or identification.
Transponders which incorporate a receiver and a transmitter are becoming increasingly used in numerous applications. Some of the more common applications include security applications where only certain equipment, or users of certain equipment may gain access to particular locations if the equipment can recognize an interrogating signal and provide an acceptable response. Examples of the transponders of the art may be seen with reference to U.S. Pat. No. 3,752,960 to Walton, U.S. Pat. No. 4,260,983 to Falck et al., and U.S. Pat. No. 4,399,437 to Falck et al. While the transponders of the art are effective for their particular purposes, they suffer from various drawbacks. In particular, many of the transponders of the art which receive at one frequency and transmit at another frequency require two or more antennae as is required in the above-listed patents. The use of two or more antennae, however, causes the transponders to be larger than what is optimal for certain applications such as wrist instruments.